The dramatic, rocky outcrops that make up the free-standing Rainbow Valley formations are part of the James Range. This breakaway escarpment is one of Central Australia's most magnificent rock formations. The spectacular sandstone cliffs of the main Rainbow Valley formation stand right on the edge of a claypan, which extends over a kilometre in length from near the park boundary. At times heavy rain fills the claypan providing stunning mirror reflections of the jagged cliffs behind it.
The area features an ancient landscape of scenic sandstone bluffs and cliffs with exposed bands of coloured rock. The colours formed between 80 and 20 million years ago when central Australia had a warm, wet climate. Generally the colours extend from the rusty, deep-red, iron-rich layers (limonite rich zone – iron oxide) at the top of the cliffs down to underlying layers of yellow and orange (less iron) and then to bleached-white sandstone (containing very little iron) at the base of the outcrops. This arid landscape is a place of dramatic change. The coloured bands radiate light that spills over the desert floor. The pastel tones of morning give way to washed out colours at noon which slowly change to a rich colour at sunset.
THE GEOLOGY:
The vivid multi-coloured bands in these formations are caused by laterisation, a process whereby underground iron oxide is dissolved by water and then drawn to the surface and higher rock levels by capillary action. The red minerals formed a dark, iron rich surface layer with the leached white layers below. The dark red capping weathers slowly, whereas the white sandstone lower on the cliff weathers quickly, breaking up into loose sand at the base. The erosion of the sandstone over millions of years has been responsible for the valley outline as we now see it, with many jagged ‘sawtooth-like’ bluffs and cliffs, eroded caverns, angular rock faces and strangely formed square towers.
In geological terms, the coloured cliffs and adjacent isolated rock outcrops are composed of Hermannsburg Sandstone estimated to be about 350 million years old. Mereenie Sandstone underlies the reserve and makes up most of the James Range to the south. Both rock types contain examples of curved and vertical joints, exposed white sandstone erosion surfaces, iron staining, honeycomb erosion and fossils. These sandstones are very soft. Their natural and archaeological features are easily marked and damaged.
LOCAL SOILS:
Sand dunes form in arid and semi-arid areas. They form and move via a few different means, all of them helped along by wind or water. Most of the Rainbow Valley Reserve’s soils are windblown sands. These soils form the sand plain and scattered sand dunes across the north and west of the reserve. In some areas, the soils are highly susceptible to erosion. These areas include the sand dunes and ridges fringing the claypans, the aprons of fine soils around the sandstone outcrops and the larger and steeper sand dunes.
WHAT'S IN A DUNE?
There are four basic types of sand dunes.
DID YOU KNOW?!
- Dunes can likely be found in any environment where there is a substantial atmosphere, winds, and dust to be blown. Extraterrestrial dunes are common on Mars and Venus and in the equatorial regions of Titan which is Saturn’s largest moon.
- Fraser island in Australia is the biggest sand island in the world.
- Mount Tempest on Moreton Bay Island is the highest sand dune in Australia. Reaching 285m, is thought to be the highest stabilised sand dune in the world.
You may log this Earthcache straight away but then please email your answers to the questions to the CO. We will contact you ONLY if your answers need extra work. Logs with no answers sent will be deleted. The answers can be found by observing the landscape and reading the information boards. Please do not post your answers in your log.
Q1. At the base of the cliffs are fallen blocks of dark red-brown sandstone. These are representative of the land surface from how long ago?
Q2. Which rock colour results when its iron is bonded with water?
Q3. How thick is the local caprock?
Q4. There are 4 large rock samples in the information shelter for you to physically handle. Rank the hardness of the four coloured samples in order from hardest to softest.
Q5. Go to S 24. 20.085 E 133 38.122 and look directly northeast. Which of the following geological features best describes what you can see on the horizon?
claypan, sand dune, pillar, single rock, arch, square tower, rock pool, crevice, valley or gorge
Q6. Go to S 24 19.981 E 133 37.980 and relax. Look out to the NE and tell us what feature you can see before you. Using your GPS, what is your elevation above/below sea level at this point?
It would be nice to see a photo of you at Rainbow Valley. Please post one if you wish.
A camping and picnic area is provided with low fees payable on site. Camping is permitted in designated areas only with facilities including gas and wood barbecues, picnic tables and a pit toilet. It is advisable to bring in your own water and firewood. Firewood must be collected outside the Reserve area.
Please remember to sign the Visitor Comments book. Remember to take only photos and leave no trace of your visit. We hope you enjoy your visit to this interesting Australian geological formation.